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Prescribed pasture burning isn’t the only step ranchers and stockmen can take this time of year to affect cattle growth while they’re on pastures later in the year, says Jessica Laurin, doctor of veterinary medicine at Animal Health Center of Marion County.

Deworming cattle in the spring has the biggest influence on gains, she said. The worm that has the biggest negative impact on cattle is Ostertagia ostertagi. In the surrounding area, the worm is dormant in the winter and becomes active again in the spring.

Mature female worms live in the abomasums — the final compartment of a cow’s stomach — and release eggs into the cow’s fecal matter. The worm eggs hatch in cow patties, then climb grass blades to be eaten by calves and continue their life cycle.

Dew that collects on grass in the spring also may contain Ostertagia larvae, making April, May, and June the months that have the greatest affect on re-infection, Laurin said.

Other worms, including Haemonchus, Cooperia, and Nematodirus also have economic impacts on cattle, but are less affected by dewormers.

There are several factors that influence how susceptible cattle are to infection.

Age and immunity: Cattle are able to build some immunity to mature worms. Animals that weigh less than 500 lbs. will not have developed the ability to suppress worm growth yet, Laurin said. A healthy, mature animal over 700 lbs. can reduce the shedding of worm eggs.

Nutritional state: Studies have shown deficiencies in caloric and protein intake can increase the presence of parasites in cattle. It is most notable in winter feeding, especially if they are milking.

Anthelmintics, aka dewormers: Dewormers come in several classes, each of which have different affects, Laurin said.

Fenbendazoles are given orally and have a duration of two to three days. They work well for cattle fed solely from a bunk and cows fed during freezing ground conditions, but will have minimal benefit for cattle on grass, she said.

Macrolide dewormers are commonly used, but Cooperia worms are developing resistance to them.

Avermectins kill many types of worms. An injected version will not kill both types of lice but will have a greater benefit against worms. Different subtypes have durations of 14 to 28 days, varying according to the type of worm affected.

Eprinomectin has a very long duration — 100 to 120 days for different types of worms — but is only approved for use on cows and calves over 300 lbs., Laurin said. Its long duration can break the worm life cycle in a pasture. It should be given as close to the time going out to pasture as possible.

Pour-on dewormers do not have as great an effect as injectible ones, but producers use them because of their ease of use and ability to kill both lice species. However, Laurin said pour-on dewormers have been implicated in speeding the development of resistant worms, especially Cooperia and Nematodirus.

“We have to be cautious that we use them appropriately,” Laurin said. “We must give them accurately for body weight or else we may contribute to resistance. So it is wise to over-dose.”

The average cow in the U.S. is 1,350 lbs. now, and pour-ons are dosed at 5 milliliters per 110 lbs., so Laurin calculates that at least 60 milliliters should be used every dose.

Ivermectin dewormers do not absorb through skin as well as others and are not absorbed much at all if the skin is wet. They are also sensitive to ultraviolet light, Laurin said, so they should never be purchased in clear bottles.

Moxidectin is available for injection or pour-on. The injection formulation is better at controlling Cooperia but doesn’t provide long lasting protection against Ostertagia at only 14 days, Laurin said.

“Implants and dewormers have the two largest effects in increasing cattle gains on grass and thus producing more beef with fewer animals,” she said. “This is the time of year when your decisions will have the greatest impact.”